The left hand pane provides the two main types of information that will be included in your Report.
The first of these is called Measures. Essentially, Measures are anything in the database that gives a Count or a Measurement. For example, Client Count is a Measurement- once you place it within your Report, the Report will return the number of Clients currently stored on the application.
You can then break down this Client Amount according to criteria such as Age or Gender. These criteria are called Dimensions. To view all available Measurements, simply double-click on the Measures heading; this will expand to display a complete list of Measures available for you to use.